Steam reforming is a well known reaction. In this reaction a hydrocarbon feedstock, such as natural gas, is reacted with steam at high temperature in the presence of a fixed bed of a steam reforming catalyst. The reactions involved are:CH4+H2OCO+3H2;   (1)CO+H2OCO2+H2; and   (2)2CO→CO2+C.   (3)The first of these reactions is known as the reforming reaction, the second as the shift reaction, and the third as the carbon reaction.
The reforming reaction is strongly endothermic and is typically carried out at a temperature of at least about 750° C. and at a pressure of from about 100 psia (about 698.48 kPa) to about 600 psia (about 4136.86 kPa). Suitable catalysts include, for example, supported nickel catalysts. The catalyst is usually packed into reformer tubes which are mounted vertically in a reformer furnace. Such reformer tubes are externally fired.
The particulate steam reforming catalyst has a lower coefficient of expansion than the metal reformer tubes into which it is loaded. Thus, when the furnace heats the reformer tubes to steam reforming temperatures, the catalyst slumps in the vertical reformer tubes because, upon heating, the walls of the reformer tubes expand more than do the catalyst particles. Then, when the temperature is later lowered, the walls of the vertical reformer tubes contract as they cool and the catalyst particles are caught as if by a tightening corset and are thereby subjected to a crushing force.
Since the steam reforming reaction involves use of high temperatures of, for example, about 750° C., the amounts by which the tube walls expand are significant. Moreover, because the catalyst particles are contained in a narrow vertical tube having, for example, a nominal diameter of less than about 6 inches (about 15.24 cm), very high crushing forces can be generated. This tends to result in attrition of the catalyst particles or in damage to the reformer tube wall. Since the vertical reformer tubes are very long and experience considerable expansion over their length during operation of the steam reformer, the catalyst particles can drop by a very significant amount inside a reformer tube but cannot rise back up inside the reformer tube when it cools due to being tightly squeezed by the cooling reformer tube, a factor that exacerbates the crushing tendency.
Repeated heating and cooling cycles lead to a deterioration in the desired characteristics of the packed bed, in that the originally loaded volume of steam reforming catalyst particles is compressed to a higher density, thereby increasing the pressure drop. In addition it has been found that increased pressure drop through a catalyst bed can be caused by, amongst other reasons, breakage of steam reforming catalyst particles resulting from incorrect charging of the catalyst or from differential expansion and contraction between the catalyst and the reformer tube due to temperature cycling at start-up and shut-down. The breakage of catalyst particles gives fragments of a smaller particle diameter, while erosion of the corners of particles gives a lower voidage due to the eroded particles packing more closely together. For further discussion reference may be made to “Catalyst Handbook”, 2nd Edition, by Martyn V. Twigg (Wolfe Publishing Ltd., 1989), at page 125. This increased pressure drop generally increases the costs associated with gas compression in the steam reforming process. Because the steam reformer furnace includes many reformer tubes providing parallel fixed beds, this can lead to increasing maldistribution, thereby causing different conversions and selectivities in different tubes. This, in turn, can lead to further problems such as carbon laydown, formation of hot spots (leading to possible tube failure and/or to sintering of the catalyst), and to development of different rates of catalyst deactivation which can further exacerbate the situation. Loss of catalyst surface material by spalling and attrition is particularly serious when the active part of the steam reforming catalyst is in the form of a shallow surface layer because in this case considerable catalyst activity can be lost or the catalyst activity can become maldistributed.
The debris from the crushing forces will accumulate in the, by now, more dense bed and also increase the pressure drop. There will be an increased likelihood of different pressure drops between different reformer tubes in the steam reformer leading to maldistribution of the reactants and reaction products. In addition, the position of the top of the bed within any individual reformer tube will be difficult to predict.
Another problem occurs in that any part of a reformer tube in a steam reformer that does not contain steam reforming catalyst is liable to overheat, with a consequent danger of tube failure, since there is no endothermic reaction being catalysed in that part of the reformer tube to absorb the radiant heat and hence to cool that part of the reformer tube. This makes it important to determine as closely as possible the position of the catalyst bed during operation of the steam reformer furnace so as to minimise the risk of tube failure through local overheating. This is a particular problem with top fired steam reformer furnaces because any slumping of catalyst in the reformer tubes will tend to result in the upper ends of the reformer tubes being uncooled by the endothermic steam reforming furnace at a point where the flame temperatures can be highest.
There is, therefore, a need in the art to provide a steam reformer reactor design which overcomes the problems associated with crushing of the steam reforming catalyst particles when the reactor is subjected to temperature cycles of heating to high temperatures followed by cooling again, and which allows low pressure drop through the catalyst bed, minimises pressure drop build-up, and allows the position of the bed to be fixed with a high degree of certainty so as to minimise the risk of tube failure.
This need has been recognised previously and there are various examples in the prior art of attempts to overcome the problems outlined above.
The crushing of catalysts by radial forces due to wide temperature cycles in tubular reactors, such as steam reforming reactors, has been recognised in U.S. Pat. No. 4,203,950 (Sederquist). In this document it is proposed that the catalyst should be arranged in an annulus with at least one wall being flexible.
In U.S. Pat. No. 5,718,881 (Sederquist et al.) a steam reformer has segmented reaction zones with individual supports for different temperature zones, the volume of the segments of catalyst being inversely proportional to the temperature of the various zones in the reformer.
The use of flexible louvered screens to accommodate particle movement is proposed in U.S. Pat. No. 3,818,667 (Wagner). Louvers are also proposed in a catalytic converter for catalytically treating the exhaust gases from an internal combustion engine in U.S. Pat. No. 4,063,900 (Mita et al.), and in U.S. Pat. No. 4,052,166 (Mita et al.).
It is proposed in U.S. Pat. No. 3,838,977 (Warren) to use springs or bellows in a catalytic muffler to control bed expansion and contraction so as to maintain a compacted non-fluidised or lifted bed. Spring loading to maintain a bed of carbon granules tightly packed within a fuel vapour storage canister housing is described in U.S. Pat. No. 5,098,453 (Turner et al.).
A ratchet device to follow the decrease in volume of a bed but restrain back-movement of an upper perforated retaining plate is proposed in U.S. Pat. No. 3,628,314 (McCarthy et al.). Similar devices are described in U.S. Pat. No. 4,489,549 (Kasabian), in U.S. Pat. No. 4,505,105 (Ness), and in U.S. Pat. No. 4,554,784 (Weigand et al.).
Pneumatic sleeves inside a catalyst bed to restrain movement of the particulate material are proposed in U.S. Pat. No. 5,118,331 (Garrett et al.), in U.S. Pat. No. 4,997,465 (Stanford), in U.S. Pat. No. 4,029,486 (Frantz), and in U.S. Pat. No. 4,336,042 (Frantz et al.).
However, these prior art proposals are elaborate and do not solve satisfactorily the problem of crushing of particulate steam reforming catalysts which can be caused by repeated temperature cycling of a reformer tube.
Catalysts, for use in the steam reforming of hydrocarbons, are usually passed over a screen to remove dust and broken pieces either before shipment and/or before loading in the reformer tubes of a steam reformer. Such removal of dust and broken pieces of steam reforming catalyst is desirable in order to minimise the pressure drop across the reformer tubes caused by the catalyst bed. This screening step constitutes a costly procedure both in terms of finance and time. Once loaded, catalyst particles usually cannot be re-arranged and the packed density only tends to increase.
The loading of catalysts into vertical steam reformer tubes can be achieved by a number of methods to reduce breakage and damage caused by free fall loading. For example, “sock” loading can be used in which the catalyst is put into long “socks”, usually made of fabric, which are folded or closed at one end with a releasable closure or tie which can be pulled to release catalyst when the sock is in position. Another method utilises wire devices or wires in tubes which reduce falling velocities. One option is to utilise one or more spirals of wire inside the tube so that the catalyst particles bounce their way down the tube and do not undergo free fall over the full height of the tube. As the tube is filled, so the wire is withdrawn upwardly, optionally with vertical fluctuations. Such devices are proposed, for example, in U.S. Pat. No. 4,077,530 (Fukusen et al.).
A further possibility is to use a line having spaced along its length a series of brush-like members or other damper members and to withdraw the line upwardly as the catalyst particles are fed into the tube, as described in U.S. Pat. No. 5,247,970 (Ryntveit et al.).
Each method of loading produces fixed beds with different bulk densities. The density differences can be quite marked.
In some applications it is desirable to maximise the amount of catalyst loaded, despite increased pressure drop through the fixed bed, in which case loading into liquid may be used and/or the tubes may be vibrated.
U.S. Pat. No. 5,892,108 (Shiotani et al.) proposes a method for packing a catalyst for use in gas phase catalytic oxidation of propylene, iso-butylene, t-butyl alcohol or methyl t-butyl ether with molecular oxygen to synthesise an unsaturated aldehyde and an unsaturated carboxylic acid in which metal Raschig rings are used as auxiliary packing material.
In U.S. Pat. No. 5,877,331 (Mummey et al.) there is described the use of a purge gas to remove fines from a catalytic reactor for the production of maleic anhydride which contains catalyst bodies. In this procedure the purging gas, such as air, is passed through the catalyst bed at a linear flow velocity sufficient to fluidise the catalyst fines but insufficient to fluidise the catalyst bodies. At column 15 lines 16 to 18 it is said:                “In order to prevent fluidization or expansion of the catalyst bed during further operation of the reactors, and in particular to prevent the catalyst bodies in the fixed catalyst bed from abrading against one another or against the tube walls, a restraining bed comprising discrete bodies of a material substantially denser than the catalyst was placed on top of the column of catalyst in each tube of the reactors.”It is also taught that this upflow removes fine particles which, if left in the densely packed vessel, may contribute to plugging of the bed.        
In U.S. Pat. No. 4,051,019 (Johnson) there is taught a method for loading finely divided particulate matter into a vessel for the purpose of increasing the packing density by introducing a fluid medium counter-current to the downward flow of the finely divided particulate matter at a velocity selected to maximise the apparent bulk density of the particulate matter in the vessel. It is taught that this method also provides a method of removing undesirable fine particles which, if left in the densely packed vessel, might contribute to plugging of the bed.
Vibrating tubes with air or electrically driven vibrators and/or striking with leather-faced hammers is described in the afore-mentioned reference book by Twigg at page 569, the latter being used to further compact the catalyst in those tubes showing low pressure drop in multi-tube applications, in order to achieve equal pressure drops in each tube.
An upflow tubular steam reformer is described in U.S. Pat. No. 3,990,858 (O'Sullivan et al.). In this proposal fluidisation of the particulate material in the catalyst tubes is prevented by providing a weighted conically shaped hollow member which rests on top of the bed of particulate material. This conically shaped hollow member is provided with elongated slots whereby fluid exiting from the bed flows into the interior of the hollow member, through the slots and into the tube outlet.
Steam reformers have traditionally been extremely large items of equipment However, in more recent years more compact designs of steam reformer have been developed. Examples include U.S. Pat. No. 4,098,587 (Krar et al.), U.S. Pat. No. 4,098,588 (Buswell et al.), and U.S. Pat. No. 4,098,589 (Buswell et al), as well as International Patent Publication No. WO 99/02254 (BP Exploration Operating Company Limited et al.) and U.S. Pat. No. 5,567,398 (Ruhl et al.). This last mentioned specification recommends the use of long thin flames to heat the reformer tubes in order to avoid excessive heating of the reformer tubes and provide high reformer tube life expectancy.
A slab reformer design is described in U.S. Pat. No. 4,430,304 (Spurrier et al.).
In U.S. Pat. No. 5,776,421 (Matsumara et al.) there is taught a reforming reactor which includes a reforming chamber in which are disposed a plurality of gas flow passages with reforming blocks containing reforming catalysts, with the locations of the reforming blocks in adjacent gas flow passages differing along the gas flow path so that adjacent reforming sections are staggered in the flow direction.
U.S. Pat. No. 4,292,274 (Faitani et al.) is concerned with a burner arrangement for a catalytic reaction furnace.
There is a need to obviate in a simple and reliable way the problems caused by crushing or attrition of particulate steam reforming catalysts, which are subjected to cycling between high and low temperatures in reformer tubes of a steam reformer furnace, especially those with small diameter tubes such as are used in compact reformer designs. There is a further need to provide an improved method of packing the reformer tubes of a steam reformer furnace with a steam reforming catalyst. There is a still further need to provide a novel method of packing the reformer tubes of a steam reformer furnace with a steam reforming catalyst in which the risk of generating catalyst fines during the loading procedure is substantially obviated, thereby avoiding inadvertently increasing the pressure drop across a reformer tube due to undesirable amounts of small particulate material. Additionally there is a need to provide a novel tubular steam reformer in which the risk of an unwanted increase in pressure drop as a result of catalyst attrition is substantially obviated. There is a yet further need to provide a design of tubular steam reformer in which the pressure drop across the charge of catalyst in each of the tubes remains substantially the same as that across each of the other reformer tubes throughout the operating life of the catalyst.